It is well known to vary the pitch of the blades on an aircraft propeller, for example in order to maintain the rotational speed of the propeller within close limits. This in turn provides improved performance over a wide range of flight conditions, and enables thrust reversal during landing and ground manoeuvring of the aircraft. It also importantly allows for the feathering of the propeller in the event of an engine shutdown in flight.
It is known to use a pitch lock mechanism to lock the propeller blades in their current position in the event of a failure of the blade pitch adjusting mechanism.
When an engine is shutdown in flight, an unfeathered propeller can present a large and flat surface to the oncoming airflow if the propellers have a hydraulic failure and do not have a pitch lock mechanism. This will cause a large drag force on the aircraft which can result in a loss of control of the aircraft.
Even where a pitch lock mechanism is provided, a failure in the propeller blade pitch control system may cause the aircraft to crash, and will invariably result in the aircraft mission being aborted.
It is desirable to provide a propeller pitch adjusting mechanism for a contra-rotating propeller system that provides a measure of control over propeller blade pitch in the event of the failure of blade pitch control for one of the propellers.